I would imagine that the only way to know for sure is to just try it. You can look up other members who might live near you who have a one. Try the link below ;https://www.gatewaytoairguns.org/GTA/index.php?action=mlist;sa=search
The Optimus belongs to the B19 family of Chinese rifles. Many of the Gamo rifles are of this platform as well so the odds are pretty good it will fit with little or no alteration. I would NOT order the Optimus trigger guard, however. It is a brittle plastic. The guard for the Benjamin Royal fits in its place and is metal. I can provide the Crosman part number if you wish. Hopefully your existing trigger guard screws will fit. Crosman and Gamo may use the same screws for the guard but I don't know for sure.
For screws, you might actually just go wander the hardware aisle at your local Ace or Valu. I've found a good lot of what I want or need in their metric fastner section. Take yours with you and match them up, then just get longer or shorter as needed.Just a guess on my part, but the front ones are probably something like M5x14 and maybe grab some 5mm flat washers to stick between the star washers and the stock if your rifle has nothing but star washers on it. Will save your stock whether wood or synthetic from getting chewed up by them.
Cool. What did you do for an end cap for the receiver?